1. The Field of the Invention
The present invention relates to methods of etching by use of maskless techniques. In particular the present invention relates to methods of etching films under conditions that cause substantially vertical etch side walls into the film. More particularly, the present invention relates to methods of infusion and effusion of an etch solution droplet as it rests upon the film to be etched.
2. The Relevant Technology
In the microelectronics industry, a substrate refers to one or more semiconductor layers or structures which includes active or operable portions of semiconductor devices. The term substrate assembly is intended herein to mean a substrate having one or more layers or structures formed thereon. As such, the substrate assembly may be, by way of example and not by way of limitation, a doped silicon semiconductor substrate typical of a semiconductor device.
In the microelectronics industry, films are required for such structural operations as gate oxide layers, etch barriers, polysilicon layers, and epitaxial layers.
FIG. 1 depicts the wetting of an etching solution droplet 10 on a film 20. In the droplet, an angle known as .theta. or the contact angle, forms between the plane of the solid surface to be wetted and a tangent to the perimeter of the liquid contacting the solid surface. In describing the forces at a solid-liquid-gas triple point interface 12, three surface tensions must balance in a static state. The surface tension between the solid and the gas, .gamma..sub.sg, is usually very small relative to the other surface tensions. In FIG. 1 the surface tension of the solid and gas, .gamma..sub.sg, is depicted as a vector 14 at solid-liquid-gas triple point interface 12. The surface tension of the solid and liquid, .gamma..sub.sl, is depicted as a vector 16 at solid-liquid-gas triple point interface 12. The surface tension of the liquid and the gas, .gamma..sub.lg, is depicted as a vector 18 that forms an angle, .theta. with the solid surface. A force balance around solid-liquid-gas triple point interface 12 reveals that EQU .gamma..sub.sg -.gamma..sub.Sl =.gamma..sub.lg cos .theta.. (1)
This expression can be rearranged to be solved for the contact angle .theta. as EQU cos .theta.=(.gamma..sub.sg -.gamma..sub.sl)/.gamma..sub.lg. (2)
FIG. 2 illustrates the interplay between surface tension of the liquid in the gas and surface tension of the solid in the liquid where the surface tension of the solid is held constant. If the surface tension of the liquid in the gas is high, an obtuse angle, .theta..sub.1 is formed. If the surface tension of the solid in the liquid exactly equals the surface tension of the solid in the gas then the contact angle is a right angle, .theta..sub.2 and the surface of the solid is neutral to hydrophobicity or hydrophilicity. If the surface tension of the liquid in the gas is low enough, an acute angle .theta..sub.3 is formed and the surface of the solid is hydrophilic to the liquid. Equation 2 does not apply when complete wetting occurs such that .theta..sub.3 is zero degrees and .gamma..sub.sg &gt;.gamma..sub.sl +.gamma..sub.lg, and does not apply when there is no wetting at all such that .theta..sub.1 is 180 degrees and .gamma..sub.sl &gt;.gamma..sub.sg +.gamma..sub.lg.
During etch of a thin film in preparation for thin film measurement, there exists an etchant surface tension problem in the prior art that results in a poor step etch. FIG. 3 illustrates an etching solution progression for an etching solution droplet 10, positioned upon film 20 which is upon substrate 30. The etching solution progression is depicted for droplet 10 as viewed in four sequential instances, from left to right. In the first instance depicted at the left, etching solution droplet 10 is seen at the instant etching solution droplet 10 makes contact with film 20. Etching solution droplet 10 is characterized by a right contact angle .theta.. Upon the commencement of etching, the chemical makeup of etching solution droplet 10 changes where chemical action of etching alters the chemical makeup of etching solution droplet 10. As film 20 dissolves into etching solution droplet 10, the chemical makeup of etching solution droplet 10 becomes changed by dilution of etch products of film 20 into etching solution droplet 10.
As the chemical makeup of etching solution droplet 10 changes, one parameter that changes is the surface tension of etching solution droplet 10. Where surface tension drops, wetting increases and contact angle .theta. decreases. As contact angle .theta. decreases, wetting causes the original footprint of etching solution droplet 10 to increase as seen by the progression of illustrations in FIG. 3. As the original footprint increases, the etch also continues to penetrate film 20.
FIG. 4 illustrates a good step etch of film 20 in which an etch between the upper surface 24 of film 20 and the exposed upper surface 26 of the substrate 30 is accomplished with minimal sloping of etched walls 22. Although some sloping may be inevitable, it is desirable to achieve minimal sloping as illustrated in FIG. 4 by the distance D.sub.1. The prior art problem of maskless diagnostic etching is illustrated in FIG. 5 in which an etch between upper surface 24 of film 20 and exposed upper surface 26 of substrate 30 depicts substantial sloping of etched walls 22. The distance between unetched upper surface 24 of film 20 and the last remaining portion of film 20 is illustrated by the distance D.sub.2. The result of a decreasing surface tension of etching solution droplet 10 is a poor step etch of film 20 that is characterized by substantial sloping of etched walls 22 in film 20.
Any attempt to measure the thickness of film 20 is hindered by substantial sloping of etched walls 22. Substantial sloping of etched walls 22 causes a non-discrete step between upper surface 24 of film 20 and exposed upper surface 26 of substrate 30. Where a poor etch step has occurred in film 20, it is difficult to measure the thickness of film 20 with existing measurement devices and techniques such as with a stylus micrometer. In contrast, a sharp etch step produced by an anisotropic etch provides a more discrete difference in height between the upper surface 24 of film 20 and exposed upper surface 26 of substrate 30.
What is needed is an etching solution droplet that maintains its optimum surface tension qualities and thereby provides an etch that is uniform in surface tension and chemistry throughout the etch.
In connection with an etching solution that will maintain its optimum wetting qualities, what is also needed is a method of using an etching solution that will not continue its etching action once the surface to be etched is etched down to an etch stop.